1. Field of the Invention
The invention relates to a novel macromolecular material displaying ionic conduction, which may be used as a solid electrolyte in an electrochemical generator.
2. Description of the Prior Art
The present invention relates to macromolecular materials displaying ionic conduction. These materials are non-crystalline at ambient temperature or sufficiently non-crystalline so that they may be employed in electrochemical generators of electricity (either primary or secondary) operating at ambient temperature.
According to the prior art, solid electrolytes are known which are comprised of solid solutions of ionic compounds in polymers. An example which may be cited can be found in European Patent Publication No. 0 013 199, entitled "Electrochemical Current Generators, and Novel Materials for Producing Such Generators." This application in particular describes solid solutions of lithium salts in aprotic polar polymers in which the polymers solvate the lithium ion. Among the polymers mentioned in said European Patent Publication are polyoxyethylene compounds, polyoxypropylene compounds, and copolymers of these with other monomers.
In order to improve the mechanical properties of the electrolytes comprised of these substances and to improve their behavior with respect to crystallization, it has been proposed to employ a complex crosslinked elastomer material as an electrolyte (French Patent Publication No. 2 485 274 (U.S. Pat. No. 4,357,401), "Solid Electrolyte Based on a Macromolecular Material Displaying Ionic Conduction"). Crosslinking is effected by use of polymeric materials having cross-linkable functionalities employing the usual techniques. According to French Patent Publication No. 2 485 274, the most usual functionalities are isocyanate groups which lead to the formation of urethane linkages. Such resins however, have the disadvantage that they can react with the metal of the anode and thus cause degradation of the polymer resulting in loss of its elastomeric pliancy. Reaction with the anode can also result, via irreversible oxidation, in consumption of a portion of the anode metal.
According to French Patent Publication No. 2 493 609, "Solid Macromolecular Electrolyte for a Lithium Electrochemical Generator", the solid electrolyte comprises as the solvated salt a polyester-type elastomeric copolymer produced by the polycondensation of dimethylterephthalate with a polyglycol and a diol. That Application mentions polybutylene glycol, polyethylene glycol, and polypropylene glycol as suitable glycols. However, those polymers suffer similar drawbacks to the French Patent Publication No. 2 485 274 since the condensates include ester functions, which also have a tendency to react with the metal of the anode and thereby degrade the polymer.
In addition, according to these two cited methods it is necessary to use stoichiometric quantities of the condensing or crosslinking agent for the intended reactions to follow their proper course. This limitation makes it difficult to eliminate the residual OH groups surviving from the initial polyols. Accordingly, the resulting macromolecular material comprises protic functions which can accelerate the reduction of the esters and urethanes at the locus of the material of the negative electrode.
In order to eliminate these protic functions, it is possible to introduce an excess of the condensation agent to eliminate the OH groups. In the case of the polycondensates obtained according to the Specification in French Patent Publication No. 2 493 609, the elimination of the OH groups leads to a substantial reduction in molecular weight of the polymer and thus a degradation of mechanical properties. In the case of the crosslinking described in French Patent Publication No. 2 485 274, the use of the technique is attended by the risks of introducing an excess of a reagent which is unstable in redox reactions.
Further, it is noted that in the case of the crosslinking according to French Patent Publication No. 2 485 274 one obtains crosslinked resins without the possibility of obtaining a thermoplastic polymer. In the case of the polycondensates of French Patent Publication No. 2 493 609 one obtains linear chains which are not crosslinked.
A need exists therefore for a macromolecular material which displays ionic conduction, which is non-crystalline or sufficiently non-crystalline and which does not react with the metal of the anode of electrochemical generators.